The problem of optical feedback affecting lasers and laser interferometers has been known for at least two decades and is noted and described in a paper by N. Brown in the November 1981 issue of Applied Optics, Vol. 20, No. 21 at pages 3711 to 3714. When the configuration of laser interferometer optics and the distance between those optics becomes such that laser light, which originates from the cavity inside the laser, becomes re-directed and re-enters the laser cavity, then optical feedback can occur. Such feedback then detrimentally affects the properties of the original laser beam, possibly including but not limited to, its intensity, its frequency, and its phase. When the distance between the optics is relatively long--for example, 50 feet or more--then, due to the natural divergence of the laser light beam and/or slight beam redirection caused by changes in atmospheric refractive index, the problem often becomes severe.
Some kinds of polarization isolation have been proposed to mitigate optical feedback in some interferometer systems by suppressing reflections which directly retrace the path of the outgoing beam; however, they are not suitable for use with laser interferometers such as the modified Michelson interferometer, which uses offset outgoing and returning beams. The paper by Brown suggests some such approaches, and approaches with other kinds of laser interferometers have been described in a paper by C. N. Man, et al. in The Journal of Physical Electronic Science Instrumentation (J. Phys. E. Sci. Instrum.), Vol. 11, 1978, pages 19-21. All these approaches fail when applied to laser interferometers such as the modified Michelson interferometer, because they block the entire return beam.
Two principal mechanisms which can bring about optical feedback are: (1) diffraction and (2) atmospheric beam deviation. They both arise from the fact that in typical configurations of interferometer optics, there frequently exists, originating from the laser cavity, an outgoing laser beam which is positioned close to and parallel to a returning laser beam traveling in the opposite direction, often but not always aiming toward a receiver. In such an optical configuration, diffraction can bring about optical feedback, as follows: If the distance that these parallel beams travel is sufficient, the natural divergence of the returning laser light due to diffraction may cause the diameter of the returning laser beam to increase to the point that it partly or entirely encompasses the outgoing beam. When this occurs, the portion of the returning beam which is sufficiently collinear with the outgoing beam may travel along the path of the outgoing beam until it re-enters the laser cavity. When the intensity and the alignment of this re-entered light are sufficient, this light interferes with the electromagnetic fields inside the laser cavity; this condition is known as optical feedback. The consequences are that the properties of the outgoing laser beam are then degraded, as described above.
In the same optical configuration, atmospheric beam deviation can bring about optical feedback as follows:
When refractive index fluctuations occur in the region traversed by the laser beams, outgoing or returning, the beams may be temporarily angularly deviated. For example, such index fluctuations may arise routinely in air as a result of turbulence, temperature gradients, pressure gradients, humidity gradients, concentrations of chemical vapors, or any combination of these effects. When the returning laser beam sufficiently deviates from its intended path, some of its intensity may re-enter into the laser cavity and may cause optical feedback. The longer the beam path, the less angular deviation that is required to cause re-entry into the cavity; also, the larger the returning beam's diameter, which increases with distance due to the aforementioned divergence, the less angular deviation that is required to cause the same effect. Consequently, when the distance between optics is relatively large, the problem may become severe.
The result of optical feedback is modulation of the output beam from the laser cavity, causing frequency and intensity fluctuations in the output laser beam. If these fluctuations cause errors in the interferometer measurement system, the measurement may cease entirely or an error message may be generated.
An object of the invention is to prevent the optical feedback by preventing a suitably directed returning beam from reaching the laser cavity and influencing the emitted laser light beam, while allowing the returning beam to be used in making the actual measurement for which the interferometer is used.
Another object of the invention is to prevent optical feedback due to the returning beam while introducing no substantial degradation of any optical properties of the portions of the returning beam needed to make a measurement, for example, the portions desired to reach any receiver. Such optical properties may include but are not limited to, intensity, frequency, and phase.
The optical feedback isolator of this invention functions in conjunction with the polarization effects of the interferometer optics, which include a polarizing beamsplitter to supply a linearly-polarized laser beam of certain polarization orientation, and a retroreflector to redirect the laser beam to the beamsplitter and reverse the handedness of the circularly-polarized light.
The optical feedback isolator comprises a pair of quarter-wave retarders.
The first retarder is inserted into the path of the outgoing laser beam, between a polarizing beamsplitting means and a retroreflecting means, with its fast axis at either +45 or -45.degree. with respect to the polarization plane of the incident beam. This converts the outgoing beam's linear polarization into circular polarization of a certain handedness. The returning beam will now be circularly-polarized with opposite handedness compared to the outgoing beam, due to the handedness reversal caused by the retroreflecting means. If any of the returning light is positioned such that it passes back through the first retarder, its polarization becomes linear with an orientation that is orthogonal to that of the outgoing beam. When it reaches the polarizing beamsplitting means, this returning light will not be transmitted toward the source of the outgoing beam, but will be reflected in some other direction. Thus, this portion of the returning beam will not reach the laser cavity, and optical feedback is thereby prevented.
A second retarder is inserted into the path of the returning laser beam, also between the polarizing beamsplitting means and the retroreflecting means, with its fast axis orthogonal to that of the first retarder. The consequence of this is that the polarization of the returning beam will be converted to linear in the original orientation of the outgoing beam. Note that this portion of the returning beam cannot cause optical feedback because it is not coaxial with any portion of the outgoing beam. When this returning light reaches the polarizing beamsplitting means, it will follow the original path, i.e., to the receiver, just as it would in the absence of any retarders at all. Thus, the isolator of the invention leaves this portion of the returning beam substantially unaffected in its optical properties.
In this invention, the first retarder and the second retarder are mutually shaped and positioned such that: 1) the entire outgoing light beam passes through the first retarder only; and 2) the portion of the returning light beam intended for the receiver passes back through the second retarder only, and 3) the undesired portion of the returning light beam, which is to be optically isolated from reaching the laser cavity, passes back only through the first retarder. Generally, if the first and second retarders are contiguous and coplanar, then most configurations which meet the requirement 1) will automatically satisfy requirement 3). In many configurations, the optimum location of the pair of retarders is as close as possible to the laser cavity while remaining between the beamsplitter means and a retroreflector means.